PRESCRIBING, RECORDING AND REPORTING PHOTON BEAM THERAPY

ICRU REPORTS 50 AND 62

Dr Mayur Mayank02.05.2011

STEPS IN RADIOTHERAPY

When delivering a radiotherapy tretament, parameters such as volume and dose have to be specified for different purposes: prescription, recording, and reporting. It is important that clear, well defined and unambigous concepts and parameters are used for reporting purposes to ensure a common language between different centers.

ICRU 50 (1993): 1993 to 1999

ICRU 62 (1999): 1999 to till date

INTERNATIONAL COMISSION ON RADIATION UNITS AND MEASUREMENTS ( ICRU )

AIM OF THERAPY

1. Radical treatment of Malignant disease

2. Palliative treatment of Malignant disease

3. Non-malignant diseases

Volumes defined prior to treatment planning :

- Gross Tumor Volume (GTV)- Clinical Target Volume (CTV)

Volumes defined during the treatment planning :

- Planning target Volume (PTV)- Organs at risk- Treated Volume- Irradiated Volume

ICRU 50

GROSS TUMOR VOLUME ( GTV )

• Gross palpable or visible/demonstrable extent and location of the malignant growth.

• It consists of :- Primary tumor- Metastatic lymphadenopathy- Other metastasis

• Corresponds to those parts of the malignant growth where the tumor density is largest. • If the tumor has been removed prior to radiotherapy then no GTV can be defined.

• GTV can be determined by using either clinical examination (inspection, palpation) and by various imaging techniques (X-rays, CT, MRI etc.)

• Method used for determination of GTV should meet the requirements for staging the tumor according to the clinical TNM (UICC and AJCCS).

Reasons for identification of GTV :

1. An adequate dose should be delivered to the whole of GTV to obtain local tumor control in radical treatments.

2. To allow for recording of tumor response in relation to dose and its variation, and to other relevant factors.

GTV

CLINICAL TARGET VOLUME ( CTV )

• It is a tissue volume that contains a GTV and/or subclinical microscopic disease, which has to be eliminated.

• This volume has to be treated adequately in order to achieve the aim of therapy : cure or palliation.

• The delineation of this volume requires consideration of factors like local invasive capacity of the tumor and its potential to spread to different regions ( eg: regional lymph nodes).

CTV

The delineation of GTV and CTV are based on purely anatomic-topographic and biological considerations without regard to technical factors of treatment.

PLANNING TARGET VOLUME ( PTV )

• It is a geometrical concept, and is defined to select appropriate beam sizes and arrangements, taking into consideration the net effect of all possible geometrical variations, in order to ensure that the prescribed dose is actually absorbed in the CTV.

• It is used for dose planning and for specification of dose.

• It has to be clearly indicated on sections used for dose planning and the dose distribution to the PTV has to be considered to be representative of the dose to the CTV.

PTV

TREATED VOLUME• It is a volume enclosed by an isodose surface, selected and specified by the radiation oncologist as being appropriate to achieve the purpose of treatment ( tumor eradication or palliation ).

• It may closely match to the PTV or may be larger than the PTV.

• If, however, it is smaller than the PTV, or not wholly enclosing the PTV, then the probability of tumor control is reduced and the treatment plan has to be reevaluated or the aim of the therapy has to be reconsidered.

Reasons for identification of Treated Volume are :

1. The shape and size of the Treated Volume relative to the PTV is an important optimization parameter.

2. Also, a recurrence within a Treated Volume but outside the PTV may be considered to be a “true”, “in-field” recurrence due to inadequate dose and not a “marginal” recurrence due to inadequate volume.

IRRADIATED VOLUME ( IrV )

• It is that tissue volume which receives a dose that is considered significant in relation to normal tissue tolerance.

• It depends on the treatment technique used.

ORGANS AT RISK ( OR )• These are normal tissues whose radiation sensitivity may significantly influence the treatment planning and/or prescribed dose.

• They may be divided into 3 classes :

1. Class I : Radiation lesions are fatal or result in severe morbidity.

2. Class II : Radiation lesions result in mild to moderate morbidity.

3. Class III : Radiation lesions are mild, transient, and reversible, or result in no significant morbidity.

ICRU 50

Irradiated Volume

Treated Volume

Planning Target Volume (PTV)

Clinical Target Volume (CTV)

Gross Tumor Volume (GTV)

ICRU REPORT 62 ( Supplement to ICRU REPORT 50 )

• Gives more detailed recommendations on the different margins that must be considered to account for anatomical and geometrical variations and uncertainties.

• Introduces a Conformity Index ( CI )

• Gives information about how to classify Organs at Risk.

• Introduces a Planning Organ at Risk Volume ( PRV )

• Gives recommendations on graphics.

Volumes defined prior to treatment planning :

- Gross Tumor Volume (GTV)- Clinical Target Volume (CTV)

Volumes defined during the treatment planning :

- Planning target Volume (PTV)- Treated Volume- Irradiated Volume- Planning Organ at Risk Volume

(PRV)

ICRU 62

Same as ICRU 50

INTERNAL MARGIN ( IM ) AND INTERNAL TARGET VOLUME ( ITV )• It is the margin given around the CTV to compensate for all variations in the site, size and shapes of organs and tissues contained in or adjacent to CTV.

• These may result from respiration, different fillings of the bladder and rectum, swallowing, heart beat, movements of bowel etc.

• These are physiological variations which are very difficult to control and result in changes in the site, size and shape of CTV.

SET-UP MARGIN ( SM )

• There can be many uncertainties ( inaccuracies and lack of reproducibility ) in patient positioning and alignment of the therapeutic beams during treatment planning and through all treatment sessions.

• These uncertainties depend on factors like :• variations in pt. positioning• mechanical uncertainties of the equipment

(sagging of gantry, collimators, and couch)• dosimetric uncertainties• transfer set-up errors from CT & simulator

to the treatment unit• human factors

SET-UP MARGIN ( SM ) is the margin that must be added to account specifically for uncertainties (inacuracies and lack of reproducibility) in patient positioning and aligment of the therapeutic beams during treatment planning and through all treatment sessions.

ITV = CTV + IM

PTV = CTV + combined IM & SM

CONFORMITY INDEX ( CI )

• It is defined as the quotient of the Treated Volume and the volume of PTV.

• It can be employed when the PTV is fully enclosed by the Treated Volume.

• It can be used as a part of the optimization procedure.

ORGANS AT RISK• According to the functional models based on the FSU (Functional Sub Unit) concept [ Withers et al., Kallman et al., and Olsen et al. ] for the purpose of evaluation of the volume-fractionation-response, the tissues of an Organ at Risk are considered to be functionally either “ serial” “parallel” or “serial-parallel” structures.

• eg : Spinal cord has a “high relative seriality” meaning that a dose above tolerance limit to even small volume of this OR may be deleterious. On the other hand, Lung has a “low relative seriality” meaning that the most important parameter is the relative size of volume that is irradiated above tolerance level.

PLANNING ORGAN AT RISK VOLUME ( PRV )

• This is a volume which gives into consideration the movement of the Organs at Risk during the treatment.

• An integrated margin must be added to the Organ at Risk to compensate for the variations and uncertainties, using the same principle as PTV and is known as the Planning Organ at Risk volume ( PRV ).

• A PTV and PRV may occasionally overlap.

GRAPHICS• These are used to delineate the different volumes and the other landmarks.

• These are in different colors for an easy and uniform interpretation.

• The convention recommended and used in ICRU 62 are:

GTV - Dark RedCTV – Light RedITV – Dark BluePTV – Light BlueOR – Dark GreenPRV – Light GreenLandmarks - Black

ADSORBED DOSE DISTRIBUTION

• The dose given to the tumor should be as homogenous as possible.

• In cases of heterogeneity of doses, the outcome of the treatment cannot be related to the dose. Also, the comparison between different patient series becomes difficult.

• However, even if a perfectly homogenous dose distribution is desirable, some heterogeneity is accepted due to technical reasons.

The heterogeneity should be foreseen while prescribing a treatment, and, in the best technical and clinical conditions should be kept within +7% and -5% of prescribed dose (Wittkamper et al., Brahme et al., Mijnheer et al.).

MAXIMUM DOSE ( Dmax )• It is the maximum dose to the PTV and the Organ at Risk.

• The maximum dose to normal tissue is important for limiting and for evaluating the side-effects of treatment.

• Dose is reported as maximum only when a volume of tissue of diameter more than 15mm is involved (smaller volumes are considered for smaller organs like eye, optic nerve, larynx).

• When the maximum dose outside PTV exceeds the prescribed dose, then a “Hot Spot” can be identified.

MINIMUM DOSE ( Dmin )

• It is the smallest dose in a defined volume.

• In contrast to maximum adsorbed dose, no volume limit is recommended when reporting minimum dose.

HOT SPOTS

• It represents a volume outside the PTV which receives a dose larger than 100% of the specified dose.

• A Hot Spot is considered significant only if the minimum diameter exceeds 15mm (in smaller organs like eye, optical nerve, larynx etc. a diameter smaller than 15mm is also considered significant).

ICRU REFERENCE POINT• It has to be selected according to the following general criteria :

- the dose at the point should be clinically relevant.

- the point should be easy to define in a clear and unambiguous way.

- the point should be selected so that the dose should be accurately determined.

- the point should be in a region where there is no steep dose gradient.

The recommendations will be fulfilled if the ICRU reference point is located :

• always at the centre ( or in the central part ) of PTV, and

• when possible, at the intersection of the beam axes.

ICRU REFERENCE DOSE

It is the dose at the ICRU Reference Point and should always be reported.

REPORTING

• It should be done in order to make exchange of information between different centers.

• It is important that the treatments performed in various centers be reported in the same way, using the same concepts and definitions.

• According to the recommendations of ICRU, as a basic requirement, the following doses should always be reported :

• the dose at ICRU reference point

• the maximum dose to the PTV

• the minimum dose to the PTV

Additional information which is considered as relevant should also be added. This could be related to :

• a more accurate and detailed description of dose distribution e.g., average dose and its standard deviation, dose - volume histograms (DVH) etc.

• an accurate description of the dose at different anatomical sites (including Organs at Risk).

Reporting these additional information ultimately contributes to the developments and improvements in Radiotherapy.

CONCLUSIONS• Proper identification and delineation of GTV is the most important factor in treatment.

• Other volumes like CTV, PTV, ITV should also be properly delineated.

• The errors like set-up error and human errors should be kept to a minimum.

• Dose prescription, fractionation and calculation should be done in the same way by all the different centers throughout the world for the proper exchange of information and reporting.

DISCUSSION